The present technology relates to heterojunction field effect transistors (HFETs) (including high electron mobility transistors (HEMTs) or metal-insulator-semiconductor-HFETs (MISHFETs) or double-channel HFETs/HEMTs/MISHFETs or dual-Channel HFETs/HEMTs/MISHFETs, or thin-body (SOI, finFET, tri-gate, gate-all-around, etc.) HFETs/HEMTs/MISHFETs), which may be used, for example, as switching devices. Such devices are typically formed of III-V semiconductors and achieve very high mobility by having an undoped channel region. In conventional HFETs, the device is described as “normally on”; i.e. the threshold voltage, also referred to as pinch-off voltage, is zero or negative, and the channel conducts electric current with little or no bias applied between source and gate. For power electronics applications, a normally off device is strongly preferred, for safety, energy conversion and circuit design reasons. For example, a normally on device will allow a significant amount of power to flow between source and drain in the event of a failure leading to a floating or grounded gate terminal.
Approaches have been tried to change the threshold voltage of a HFET. In “Recessed-Gate Structure Approach Toward Normally Off High-Voltage AlGaN/GaN HEMT for Power Electronics Applications,” Saito et al., IEEE Transaction on Electron Devices, Vol. 53, No. 2, February, 2006, pp. 356-362, the authors describe thinning the barrier under the gate to increase the threshold voltage. This approach increases fabrication complexity, requiring extra etching and cleaning steps, and etch damage may result. The thinner barrier and etch damage increase gate leakage. The etch may not be uniform, so the resulting devices may not have uniform threshold voltages.
“A Normally-off GaN FET with High Threshold Voltage Uniformity Using a Novel Piezo Neutralization Technique,” Ota et al., IEDM 2009, pp. 153-156, describes a recessed-gate HFET in which a “piezo neutralization (PNT) layer” is formed at the bottom of the gate recess to improve threshold voltage uniformity. Formation of the PNT layer increases fabrication complexity and cost significantly, requiring formation by metal organic chemical vapor deposition (MOCVD) of three rather than one barrier layer of AlxGa1-xN, barrier etch, and atomic layer deposition of a gate oxide.
“Enhancement-Mode Si3N4/AlGaN/GaN MISHFETs,” Wang et al., IEEE Electron Device Letters, Vol. 27, No. 10, October 2006, pp. 793-795, describes a method in which plasma treatment of the gate and a two-step Si3N4 deposition process increase threshold voltage. The plasma treatment will cause damage, and fabrication is more complex.
“Gate Injection Transistor (GIT)—A Normally-Off AlGaN/GaN Power Transistor Using Conductivity Modulation,” Uemoto et al., IEEE Trans Electron Dev, 54(2007), p. 3393, describes an HFET using hole injection from a p-AlGaN gate to the AlGaN/GaN heterojunction to increase threshold voltage. As in the other approaches, extra deposition, etching, and cleaning steps are required, increasing device complexity and cost.
It is desirable to change the threshold voltage of a device without significantly increasing device cost and complexity. Altering threshold voltage in order to produce an HFET that is normally off, or altering threshold voltage in some other way, may be advantageous.